Low back pain is responsible for approximately 90 billion dollars annual costs in the United States. Previous studies of disc structural integrity and mechanical function have identified key events in progressive disc degeneration including early loss of nucleus pulposus (NP) glycosaminoglycan content (which decreases NP pressure) progressing to annulus fibrosus (AF) radial and circumferential tears. While AF tears are thought to occur in response to alterations in the AF loading environment stemming from decreased NP pressure, the physical mechanisms for initiation and progression of AF tears are not understood. Very little is known about how the internal disc mechanics are affected by the state of degeneration or by physiological factors (e.g., diurnal hydration, loading rate). Moreover, local disc mechanics at the site of tears are completely unknown. We hypothesize that internal AF stresses and strains are elevated in response to reduced NP pressure and other degenerative changes affecting AF material properties, that local stress-strain concentrations occur adjacent to AF tears, and that dynamic loading further elevates the AF stress and strain. The objectives of this proposal are to quantify internal AF stress and strain and the effect of degenerative state (reduced NP pressure, AF tears), hydration, and dynamic loading on internal disc mechanics using an integrated experimental and modeling approach. Integration of experimental methods and models, which inform each other to address hypotheses for mechanical mechanisms of progressive disc degeneration, is a distinctive approach, and is our focus in the following 3 Specific Aims. Aim 1: Quantify the strain adjacent to AF tears under physiological loading of an intact bone-disc- bone unit. High-field microMRI, a custom built MR-compatible loading system, and state-of-the-art image registration will be applied to evaluate the role of degenerative state on AF stress-strain behavior and to specifically quantify strain fields around AF tears already present in the human cadaveric disc. Location, magnitude, and distribution of strains will be identified for each loading condition (compression, torsion, bending) at equilibrium. Aim 2: Develop a new disc FE model and validate it by comparing predicted and measured internal disc strains. The model will incorporate a hyperelastic biphasic constitutive material formulation that includes recently measured NP and AF tissue properties for fiber-reinforced anisotropy, nonlinearity, and osmotic swelling pressure. Aim 3: Calculate stresses and strains associated with degeneration, loading rate, hydration, and AF tears using the model from Aim 2. Incorporate degeneration by geometry and material properties and validate the effects of degeneration as done in Aim 2 for the non-degenerate disc. This work is significant in that it evaluates physical mechanisms in disc degeneration, a critical step in understanding and treating low back pain.